It is widely known that a bare silicon sample contains a large amount of surface states, at which injected or photogenerated minority carriers can recombine. Thus, for silicon based devices where transport of minority carriers are crucial for efficient operation, such as in silicon based solar cells, reduction of the surface recombination velocity by a surface passivation technique is extremely important.
Most industrially manufactured solar cells are presently consisting of a single crystalline or polycrystalline wafer of one type of conductivity, with a thin diffused layer of the other conductivity present at one surface. Atop this surface, which is exposed to light during operation, a thin hydrogenated silicon nitride film is commonly deposited to obtain some degree of surface passivation (see e.g. R. Hezel et al., Journal of Applied Physics 52, (1981) pp. 3076-3079). This film also acts as an anti-reflection coating to increase the light trapping in the device. Such silicon nitride films can be deposited by, among other techniques, plasma deposition from a source gas mixture of SiH4 and NH3. Contact metallization is then achieved by e.g. screen printing an array of contacts on the light-receiving surface as well as a back contact layer with soldering pads at the reverse surface of the cells, and electrical contacts to the silicon device is obtained by a subsequent firing process. Although the above solar cell structure is successful in achieving decent current conversion efficiencies, it is generally accepted that as the material quality of the silicon wafers increases, better passivation of surface defects is necessary in order to further increase the conversion efficiency of such solar cells.
During the past years, other cell structures that overcome some of the limitations behind the above mentioned structure have been presented. Specifically, reduction in the light shadowing at the front surface has been proposed and verified through collection of both polarity current collection terminals at the reverse surface, i.e. the surface not primarily exposed to light, of the solar cells. Roughly, methods for obtaining such reduced shadowing can be divided in two groups as described in the following. Firstly, the solar cells may incorporate a carrier collecting region on the light-receiving surface, in combination with methods to pass current through or around the substrate to a connection area on the back surface. Current can be passed from a collection grid on the light-receiving surface to the back surface around the edges of the solar cell by incorporation of metallized regions on one or several sides of the cells, often referred to as metallization wrap around (MWA), (see B. T. Cavicchi et al., “Large area wrap around cell development”, Proc. 16th European PVSEC, 1984; W. Joos et al., “Back contact buried contact solar cells with metallization wrap around electrodes”, Proc. 28th IEEE PVSC, 2000). Alternatively, current can be passed to the back surface through metallized holes (or vias) through the substrate, often referred to as metallization wrap through (MWT) when a current collection grid is present on the light-receiving surface, or emitter wrap through (EWT) when no such collection grid is present, (see U.S. Pat. No. 3,903,42, G. J. Pack; U.S. Pat. No. 5,468,652, J. M. Gee; U.S. Pat. No. 6,384,317B1, E. Van Kerschaver et al.; U.S. Pat. No. 2004/0261840A1, R. M. Schmit et al.; International Pat. No. WO 2005/006402A2, R. M. Schmit et al.). Secondly, there may be no carrier collection region on the light receiving surface, both charge type carriers being collected at current collection contact regions solely at the back surface of the cells (see U.S. Pat. No. 4,395,583, A. Meulenberg; U.S. Pat. No. 4,478,879, C. R. Baraona et al.; U.S. Pat. No. 4,838,952, H. G. Dill et al.; U.S. Pat. No. 4,927,770, R. M. Swanson).
Especially in the latter of the above mentioned techniques, i.e. solar cell structures employing no carrier collection junction at the light receiving surface, efficient surface passivation of the front surface is essential for efficient operation. In addition to surface passivation by deposition of a thin hydrogenated silicon nitride film as described above, another method to achieve efficient surface passivation used both in sensor devices and silicon solar cells is deposition of a thin hydrogenated amorphous silicon layer. Hydrogenated amorphous silicon film can be manufactured by, among other techniques, plasma deposition from a SiH4 precursor gas. In the case of amorphous silicon thin films, a technological barrier is the lack of stability of the surface passivation upon high temperature treatments, limiting subsequent device manufacturing to relatively low temperatures. In particular, for the use of amorphous silicon layers in solar cell devices, the metallization of contacts are restricted to low temperature processes. This complicates device manufacturing, and presents a barrier for implementation of amorphous silicon films as a surface passivation layer in industrial manufacturing of solar cells.